The present invention relates to a new process for the production of water-blown polyurethane foams from known starting materials and alkali metal salts of organic carboxylic acids as catalysts as well as special additives which substantially increase the compatibility of these catalysts with the other starting materials. The process is suitable in particular for the production of composite articles by back foaming plastics with such reaction mixtures. The invention also relates to the composite articles obtainable by this process.
Water-blown foams (and in particular, those which are used for so-called back foaming processes) from a reaction mixture which reacts to form a flexible or semi-rigid polyurethane foam, are of great technical importance and are used, for example, on a large commercial scale for the internal fittings of motor vehicles and aircraft and for the production of upholstered furniture. In the back-foaming process, a foamable reaction mixture is applied to a covering layer (typically, a thermoplastic film) which has been placed in a suitable mold and is allowed to foam against that layer. For most of these applications it is important that the foams, apart from having good mechanical properties, should have little or no influence on heat aging of the top layer. In addition, the raw materials used should have a constant reactivity even after prolonged storage.
The tertiary amines frequently used as catalysts in the reaction mixtures have the disadvantage of leading to poor heat resistance of the foams. It has already been attempted to replace these amines, for example, by alkali metal salts of carboxylic acids such as potassium acetate. Using these catalysts as the only catalysts has, however, the disadvantage that the time required inside the mold is too long or (in the case of high catalyst concentrations) the cream times are too short. In addition, the use of such alkali metal salts entails the problem that an irreversible exudation of salts is observed, in particular when there are fluctuations in the temperature during storage and transport of the polyol components, and this constitutes a very serious practical problem. The use of organic tin compounds as catalysts is frequently unsatisfactory due to the hydrolysis stability, which renders a constant activation over a prolonged period of storage impossible in polyol components which contain water.
German Patent 4,029,081 recommends the use of salts of hydroxycarboxylic acids which are soluble at room temperature in the usual polyol components but separate at temperatures above 40.degree. C., and form an insoluble ground sediment after cooling to room temperature.
U.S. Pat. No. 4,868,043 recommends the use of an alkali metal or alkaline earth metal salts of carboxylic acids. The acids are produced from (i) carboxylic acid anhydrides and (ii) alcohols which are free from ether groups. Preferred acids are produced from carboxylic acid anhydrides and long chain diols. The last mentioned catalysts which are preferred according to the prior publication have, however, the disadvantage that due to their monofunctionality in isocyanate addition reactions they lead to chain terminating reactions and hence frequently to difficulty in working up the reaction mixture and to foams with inferior mechanical properties. The carboxylic acids free from hydroxyl and ether groups, which according to the prior publication do not belong to the preferred catalysts, have an unsatisfactory compatibility with the other reactants. Lastly, common to both types of catalysts of the prior publication is that their use results in reaction mixtures which do not yet optimally meet the requirements in practice as regards the relationship between a desired long cream time and a desired short mold release time.
Our own earlier German Patent Applications P 42 18 840.7 and P 42 32 420.3 recommend the use of alkali metal salts of carboxylic acid esters containing at least two hydroxyl groups per molecule or carboxylic acid esters which are substantially or virtually free from hydroxyl groups as catalysts to overcome these difficulties.